There are a lot of people that have varying opinions and so I don't want to interfere with the due process - but I'll throw my thoughts in here to help you.
Well, I too get a bit busy, and still am, so forgive the delay in posting.
Ok, you have a 5-pin radio but a 4-pin setup...(let me get this straight) Opposite side? Wow...kinda' wild - why not just keep the original hole and use it for the 4-pin...?
Alrighty, tell me when you have time...
But when you "swap" those 4-pin to 5-pin and back from 5-pin to 4-pin - the grounds for, say, the MB3756 versus the Audio return are in different locations. Which pin you use - may have a trickle voltage when the radio is unkeyed.
Depending on where - (which pin) you route your ground, what you'll get in noise, is what you're asking - it's not an easy question.
If you can, make the radio a NEGATIVE Ground Only chassis by soldering a braid wire from the TOP of either your power choke transformer or the metal screw-lug combo used for the bypass caps - to BOND the negative lead at the UNIDEN-Style power connector to the Chassis and make FOIL ground the SAME as Chassis ground. A.K.A. - Strapping the radio. (You can simply unsolder the "Black wire" from the Uniden plug and resolder it past, across from - the bypass filter disc caps - to the solder lug used to solder the NEGATIVE CHASSIS side of that Negative Lead power filter cap that uses the "screw nut bolt combo" on the back of the Uniden Power Plug to complete the circuit just make sure its' the Chassis side of the cap...)
Then once you've got the chassis and foil ground together, you can use Pin 2 shield ground for this 4-pin plug idea...
If you don't bond the chassis and foil grounds together, then you will pick up noise and the radio may become unstable (squeal).
Pin 4 of the 3,4 and 5 Pin combo - Toggle ground - is the UNFILTERED foil board ground for the MB3756 switch function.- and is why the 5-pin DIN idea kept AUDIO and SWITCH functions separate - the 3756 didn't have issues, the trickle voltage from it's lines that goes to TX and RX - has a residual charge from their respecting outputs where noise can be INJECTED into the power regulator power feed lines - and is what made noise in the Mic's own Audio line.
You also had SPEAKER return too (pin 3 is from speaker) - so once the bonding is done - speaker potential differences as well as grounding for Chassis will be equalized and less likely to cause problems.
As personal preference - it's up to you - but once the radio is BONDED/Strapped - the issue becomes more of preventing a ground loop from forming and adding more noise - so If Pin 2 doesn't work - use Pin 4 instead.
And if Alternator whine shows up - then the radio needs to have SINGLE POINT grounding - get rid of the power plugs Negative Wire - cut it off - remove it - use the chassis bolts to bracket, to ground for mobile mounting . Using single wire grounding lets the chassis ground handle the eddy current that will form when you're Negative lead wire is at a different location than the radios own ground. Your radio has a filter "ring" al along the chassis board mounting locations - works well for this - use it.
Got a Permanent Antenna mount? Good ground to it? - Low SWR too? Fantastic - let that location serve as your single point ground if you can't use the bracket. Just hook up the coax and away you go!
:+> Andy <+: